In general, in a wiper device to be mounted on a vehicle, a wiper motor with speed reduction mechanism is used as a driving source for periodically swinging a wiper arm. This wiper motor with speed reduction mechanism has: a motor unit having brushes and an armature; and a speed reduction mechanism unit for reducing the speed of rotation of the armature of the motor unit. The speed reduction mechanism unit is provided with a control device for controlling the rotation of the armature, thereby controlling a position and a speed of the wiper arm. Also, the control device is provided with a plurality of field effect transistors (hereinafter simply referred to as “FETs”) as switching elements. Electric current to be supplied from a power supply to the motor unit is controlled with an ON/OFF operation of these FETs.
The FETs produce heat due to switching loss occurring at the time of this ON/OFF operation and reach high temperature when the ON/OFF operation is frequently performed. Heat generated from the FETs is transmitted together with heat from the motor unit to a control board. For this reason, on a cover of the speed reduction mechanism unit, a heat sink is provided in the vicinity of the FETs disposed on the control board of the control device, and heat generated from the FETs is dissipated by the heat sink to the outside.
However, for example, when the motor unit continuously operates, large heat is generated from the FETs and coils of the motor unit. This heat is transmitted to the control board, and we are concerned that the temperature of the control board may exceed a specified (durable) temperature. In order to get around this, a temperature detection circuit is provided on the control board, and when it is determined by the temperature detection circuit that the temperature of the control board exceeds a predetermined temperature, the temperature detection circuit is configured to cut off the electric current to be supplied from the power supply via the FETs to the motor unit. With the cutoff of the electric current, the control board is prevented from going out of order (for example, Japanese Patent Application Laid-Open Publication No. 2007-097352).
As described above, the FETs and the coil of the motor unit are a main heat source of heat to be transmitted to the control board. However, for the purpose of decreasing the size of motors, in a motor for use in a wiper device, a multipolar motor with four or more magnetic poles of a magnet has been suggested in recent years, and the brushes can also be a source of heat to be transmitted to the control board. In this multipolar motor, a distance between a brush on a ground side and a brush on a power supply side disposed in a brush accommodating part is short in some cases. As such, when the brushes on the ground side and the power supply side are close to each other, the temperature of the resin made cover provided near the brushes and integrally mounted on the speed reduction mechanism unit is increased due to heat generated at the brushes. For this reason, a temperature gradient between the cover and the speed reduction mechanism unit including the control board is increased, and the temperature of the control board is increased not only due to heat generated from the FETs and the coil of the motor unit but also due to heat generated from the brushes. And, with heat from the brushes being further added to the control board, the temperature of the control board is prone to be further increased, and the temperature of the temperature detection circuit is prone to reach a predetermined temperature, thereby posing a problem such that it is difficult to actuate the motor with speed reduction mechanism for a long period of time.
Therefore, an object of the present invention is to provide a motor with speed reduction mechanism capable of efficiently dissipating heat generated from brushes and reducing the heat transmitted to the control board from the brushes.
Additionally, in a wiper motor where the armature rotates in a forward direction and a reverse direction by using a two-pole magnet and two brushes, it is necessary to increase the thickness of the yoke on the basis of magnetic flux of the motor unit, which may increase the weight of the wiper motor, and therefore, also increase the inertia of wiper motor. Thus, there is a possibility of deteriorating the control accuracy of the wiper.
Therefore, another object of the present invention is to provide a wiper motor improved in weight and size, and improved in wiping accuracy.